1. Field of the Invention
The invention relates to a method for fabricating semiconductor device, and more particularly, to a method of forming contact plugs.
2. Description of the Prior Art
In current semiconductor industry, polysilicon has been widely used as a gap-filling material for fabricating gate electrode of metal-oxide-semiconductor (MOS) transistors. However, the conventional polysilicon gate also faced problems such as inferior performance due to boron penetration and unavoidable depletion effect which increases equivalent thickness of gate dielectric layer, reduces gate capacitance, and worsens driving force of the devices. In replacing polysilicon gates, work function metals have been developed to serve as a control electrode working in conjunction with high-K gate dielectric layers.
However, in current fabrication of high-k metal transistor, particularly during the stage when contact holes are formed adjacent to the metal gates, phenomenon such as bowing profile occurring on contact holes is often observed. This induces poor adhesion as well as poor gap-fill for metal layers later to be deposited into the contact holes. Hence, how to resolve this issue has become an important task in this field.